When a computer system is up and running the data on the disk is in transient state. There are lots of changes happening to the data directly or indirectly because of user actions. After finishing the work when that computer system is shut down the data on the disk is typically in a state where backup is highly desired so that the system can be recovered back to that same state if something goes wrong with it. Since the machine is shut down user can perform an offline backup of all data by connecting the disks of the computer to other system. But this cannot be practically done every day. More desirable solution would be to automatically perform the backup of the machine when it is shut down every time. However back up of a machine can take potentially very long time and it would block the shutdown of the system for that long.